Really sounds like heat,
Okay lets do this, what is he seeing? does he get to the boot screen? if so tell him to enter the bios fast as he can and check the temps,
Then I also need to know where are we rebooting at? what point? Are we trying to install an OS? Does he have a Sata drive?
Then from there need to look at this
Heck tell him to do this, you always should try to set up outside the case. I personally do hook it before throwing it into the case because I don't want to pull it back out if something is not working.
Be sure and always ground yourself by touching the power supply before handling the motherboard or other components to avoid electrostatic discharge. Use properly grounded outlet.
Non-conductive surface, such as a linoleum tabletop, cardboard or rubber mat.
Known good power supply.
(Access/Enter Motherboard BIOS
and Troubleshooting BIOS Beep Codes
Troubleshooting a Video Adapter and Computer Monitor Flow Chart
RAM -one RAM stick (note some mobo's require you to install a single stick in a certain slot, read the manual)
Keyboard - Not mandatory
Speaker - Hear beep codes
Power Supply -
(Flowchart for ATX Power Supply Repair
best way is just try a known good power supply)
Remove everything from the case
Connect the components listed above, check all connections and be sure they are in their proper location and secure. If your motherboard has a small, square 4 pin power connector make sure its connected,
If you have already tried the unit inside the case, re-seat the RAM and video card a few times to clean up the contacts. This means pull the card out of the slot and put it back in, often 6 or 8 times are needed to clean off oxidation.
Locate the Power On pins on the motherboard and short them briefly with a small screwdriver, that should power up the computer. If everything is properly connected, in working order and contacts are clean, you should see a BIOS screen and hear one beep.
Now we know that it may be a Mounting issue and you should check each standoff that is not matching up with a hole int the motherboard, one other thing if you have a metal hole (like in pic below, metal ring around the hole) Don't use those cardboard washers, the holes are made to ground the motherboard. only use the cardboard washers if you have no metal hole or in some cases you may need add a few washers under the motherboard (noway would you need to on every hole) to keep it from shorting out.
Or it may be one of the pieces of hardware we have not connected yet.
If you get no video or beep, switch off the power strip or unplug the power supply to shut down. Reseat video card and RAM. Sometimes this must be done several times, heavy oxidation can be difficult to clean up.
If you still get no beep or video after several tries and have reseated the RAM and video, further troubleshooting is necessary. I won't go into great detail here, but the things to check are:
Video card - try a different one.
RAM - try a different chip.
Motherboard - Have it tested by a reputable computer shop.
CPU - Have it tested by a reputable shop.
Power supply - A weak power supply can be difficult to sort out, Best way is to try a Known good PS to diagnose